Bacillus sphaericus strains have demonstrated potential as microbial control agents of mosquito larvae of major disease vector species. Before these strains can be advanced to large scale field testing and eventual widespread use in vector control, it is necessary to better understand the basis for their insecticidal activity. Bacillus sphaericus apparently produces a toxin which is closely associated with the bacterial cell. The proposed research will attempt to isolate an insecticidal fraction from the bacteria, using larval digestion, enzymic digestion, sonication, and shearing techniques. A reliable bioassay will be devised, utilizing injection into the hemocoel of large caterpillars or mosquito larvae, force feeding of mosquito larvae, and/or treatment of isolated larval digestive tracts. Centrifugal techniques will be used to separate cytoplasmic and cell wall components for determination of their activity.